A known practice for monitoring sealing in units such as tanks or vacuum installations consists in using a tracer gas such as helium, and in injecting the tracer gas into the unit under test. If the unit has a crack or other leak, helium escapes therethrough and is directed to a measuring device such as a gas analysis mass spectrometer which then enables the size of the leak to be determined as a function of the quantity of helium detected.
A conventional helium detector generally comprises four parts:
(1) A first part for evacuating air from the unit under test, eg. a primary pump of the vane type in association with items such as a pump shut-off valve, an air inlet valve, and a pressure measuring manometer;
(2) A second part for analysing the tracer gas coming from said first part, and therefore including an analysis cell such as a mass spectrometer, a primary pump such as a vane pump, a secondary pump such an ionic or turbo-molecular diffusion pump, a trap, eg. using liquid nitrogen, and a pressure measuring manometer;
(3) A third part suitable for conveying the gas mixture for analysis, or a fraction thereof, into said second part and generally including a variable flow rate valve; and
(4) A fourth part for calculating and displaying the results, and hence the size of the leaks, eg. by means of suitable electronic apparatus.
Unfortunately, in conventional leak detection techniques, a leak to be detected may be anywhere in the range 10.sup.-11 to 10.sup.3 atmosphere cm.sup.3 /second. Such a range is generally expressed as being over N decades, which means that the values indicated correspond to a range of 15 decades.
Because of this very wide range of possible leakage rates, leak detector manufacturers have been constrained to provide a variety of set-ups matched to specific, narrower ranges.
For example, for low value leakage rates, say in the range 10.sup.-11 to 10.sup.-5 atmosphere cm.sup.3 /sec, the entire helium flow is directed to the analyser, with the valve in said second part being fully open.
For medium leakage rates, say in the range 10.sup.-5 to 1 atmosphere cm.sup.3 /sec a portion of the gas flow is diverted via the primary pump of the said first part of the detector, while the valve in the second part is either partially open or else is replaced by a low flow rate valve.
Finally, at high leakage rates, ie. rates of more than 1 atmosphere cm.sup.3 /sec, it is necessary to limit the flow rate through the valve in the second part by some suitable means such as a diaphragm, a capilary tube, or the like, or even to replace the valve with a membrane.
The net result is that the analysis cell measures a helium flow rate q passing through the cell which, depending on circumstances, may either correspond to the total rate Q of helium leakage from the unit under test, or to a fraction thereof.
The electronic circuits associated with the analysis cell are capable of measuring only over a relatively small number of decades, say 5 or at most 6.
In other words, current detectors operate only over a limited number of decades, and there is no way for a user to be informed simultaneously of the exact flow rate Q through the crack being measured, and the appropriate range for performing the measurement. For example, if a user sets the apparatus to measure a leakage rate of about 10.sup.-10 atmosphere cm.sup.3 /sec when the leakage is in fact about 10.sup.-2 atmospheres cm.sup.3 /sec, the "measured" result will be erroneously limited to about 10.sup.-5 atmosphere cm.sup.3 /sec.
Preferred embodiments of the present invention remedy the above-mentioned drawbacks, and enable an operator to be fully informed about the capabilities of the measuring apparatus.